The invention relates to a blood filter that is to be positioned within a blood vessel to trap blood clots carried along by the flow of blood. It should be noted at this point that the filter according to the invention can be both a "definitive" (or permanent) filter and a "temporary" filter, that is a filter which can be implanted in a vessel and which can be left there permanently, or removed after a given period of implantation (nowadays typically of the order of a fortnight).
Conventionally, a filter known as a "definitive" implantation filter is provided with means for fastening or attachment to the wall of the vessel in which it is implanted.
Some examples of such filters are described in U.S. Pat. No. 5,059,205, U.S. Pat. No. 5,133,733 or U.S. Pat. No. 5,344,427. However, some such blood filters are also known which have hooks for attachment to the wall of the vessel and which are nevertheless defined as being removable or repositionable, as long as the period for which they have been positioned in a specific location within a vessel has not been too long and cellular development does not in practice prevent them from being moved. U.S. Pat. No. 5,324,304 describes such a filter whose head has a hook enabling the filter to be caught.
However, other filters defined as "temporary", that is removable under the conditions outlined above, are such that the filter itself is attached to a long, flexible carrier tube (often a catheter) which meanders through the body of the patient as far as the cutaneous surface region from which the filter was introduced into the vascular system, this elongate tube even generally protruding outside the patient's body. Some examples of such filters are described in U.S. Pat. No. 5,300,086, in FR-A-2 713 081 or in U.S. Pat. No. 4,969,891.
Among these different types of filter, the one according to the invention is, structurally, more especially a filter having an axis and comprising a head which is arranged substantially on that axis and to which there are attached and from which there extend several legs or limbs, comprising at least one elongate (filiform) element having a first end and a second, opposite end, the legs being radially movable between a radially expanded state and a radially contracted state in which the legs are brought close to the axis, the elongate element of at least some of these legs being in a form folded back upon itself, substantially in the form of a loop.
A filter with such a structure is disclosed in U.S. Pat. No. 5,344,427.
However, that filter, like those mentioned above, still has defects in regard to the permeability of its head in respect of blood circulation.
This is a critical problem since the filter must be capable of trapping and retaining blood clots of at least a given size, but it must not excessively disrupt the blood circulation or the vessel in which it is implanted.
These two constraints are difficult to satisfy simultaneously.
The filter's mechanical resistance, its suitability for implantation from a tube (catheter) having a small diameter and its reliability must also be taken into consideration.